This invention relates to videotex systems.
In typical videotex systems, textual and qraphic information from a variety of sources (such as newspapers, retailers, or banks) is made available by a videotex supplier to a large number of subscribers via interactive two-way communication over the switched telephone network.
The videotex supplier electronically stores the information supplied by the different sources in the form of hundreds of thousands of pages (frames) each representing a collection (arranged, for example, in rows and columns) of alphanumeric and graphic characters to be displayed on a cathode ray tube (CRT) at the subscriber's location. Each frame is sent to the subscriber as a stream of encoded bytes. Some of the bytes identify the particular characters to be displayed. Other bytes are essentially instructions about the attributes of the displayed frame, for example, the size of a particular character or the background color for a particular line of characters. The characters and attribute information are translated into the encoded bytes in accordance with a set of rules (protocol) applied by each supplier.
In addition to a CRT, each subscriber has a decoder for converting the received encoded bytes back to the displayable alphanumeric or graphic characters and attribute instructions, and a keyboard with which the subscriber can tell the videotex supplier what frames to send to the subscriber, and can respond to questions posed in particular frames.
Different videotex protocols have been developed in different countries, for example Prestel in the United Kingdom, Telidon in Canada, and NAPLPS in the United States. A description of the Prestel system is set forth in Hudson, "Prestel: The Basis of an Evolving Videotex System", (BYTE, July, 1983, p. 61), and in "Prestel Terminal Specification" updated through 8/1/83, available from Prestel Headquarters, Prestel 435, Telephone House, Temple Avenue, London EC4Y OHL, England, both incorporated herein by reference.
Some videotex decoders allow the subscriber to switch between viewing frames being supplied over the telephone line and frames which have been stored locally. Frames explaining to the subscriber how to proceed may also be displayed.